Begin by preheating your oven to 375°F (190°C). Lightly coat a 9x13-inch baking dish with butter.
In a large skillet, heat some oil over medium-high heat. Add chopped onions and minced garlic, sautéing until the onions start to become translucent, about 5 to 7 minutes.
Next, mix in diced bacon, bell pepper, and jalapeños. Cook this mixture until the bacon is nice and crispy, which should take around 7 to 10 minutes.
In a separate bowl, combine beer, barbecue sauce, brown sugar, and molasses. Add in pinto beans, the cooked bacon and vegetable mixture, along with garlic salt and black pepper. Stir everything together until well combined.
Transfer the bean mixture into the greased baking dish and spread it out evenly.
Bake in the preheated oven until the top is golden and the mixture is bubbling, approximately 45 minutes.